The present invention is directed towards a method and apparatus for determining the particle size distribution of materials in general, and in particular, a method and apparatus for determining the particle size distribution of lightweight expanded polymers, such as shredded expanded polystyrene.
In the course of erecting steel structures, a thick coating of inorganic material is commonly applied to the metallic structural elements to achieve a number of objectives including fire retardance, improved appearance and sound deadening. While several types of formulations have been applied for these purposes over the years by means of a variety of techniques, a successful system consists in spraying onto the steel surfaces settable aqueous mixes composed essentially of calcined gypsum, a lightweight inorganic aggregate material such as exfoliated vermiculite, a mixture of fibrous materials such as a high wet bulking cellulose fiber and glass fiber, and an air entraining agent.
A composition of this type is described by Bragg in U.S. Pat. Nos. 3,719,573 and 3,839,059, along with the most desirable application technique, i.e., pumping the aqueous mix and spraying it directly onto the steel in one layer.
However, vermiculite is a naturally occurring mineral which is subject to variation in quality, consistency, and uniformity. Moreover, since the vermiculite mineral must be expanded at very high temperatures prior to use, its cost is subject to unpredictable variations in energy cost.
An alternative to the gypsum-vermiculite mixes is disclosed in co-pending U.S. Patent Application Ser. No. 384,941, assigned to the assignee of the instant invention. Specifically, sprayable cementitious compositions containing shredded polystyrene having particular particle size distribution limits as a lightweight aggregate are taught as fireproofing compositions for structural steel members. Uniform consistency and quality of the compositions, defined in terms of pumpability, hangability and yield, are achieved by controlling the size distribution of the shredded expanded polystyrene in the product.
Apparatus for and methods of sieving and determining particle sizes of various substances are known. For example, U.S. Pat. No. 4,261,817 discloses sieving apparatus for use in determining the fineness of granular substances. The apparatus includes a suction chamber connected to a sieve by a plurality of chambers. A partial vacuum is applied to the chamber which pulls fine particles within the sieve through the sieve cloth into the suction chamber. Radial air inlet passages allow air to flow into the sieve case to agitate the sample.
U.S. Pat. No. 2,611,486 discloses apparatus for use in classifying granular material according to particle size. A vortex is formed in a chamber to disturb particles upon a sieve in the chamber in such a manner as to allow the small size particles to pass through the sieve.
However, heretofore no suitable method or apparatus existed for determining the particle size distribution of lightweight expanded polymers such as shredded expanded polystyrene. For example, the foregoing prior art does not address difficulties encountered in attempting to determine particle size distribution of lightweight expanded polymers, such as minimization of moisture, prevention of clogging of the apparatus, and minimization of static charge. Accordingly, the instant invention is directed towards a method and apparatus for determining the particle size distribution of such materials.